Cisco router pass through fortigate?

Hello,

we've just moved over to Fortigate101E from sonicwall for our remote offices. 

the gist of what i'm trying to do is as follows. some of our offices have a modem with one active port, which i'm using for the fortigates public IP. we also have a cisco voice router on site that is also assigned with a public ip. however with one available port on the modem take by fortigate, can the cisco router pass through the fortgate and out to the internet as if it were connected to the modem?

alternatively we can just put in a switch before the fortigate so they can bother use there public IP, however we wondered if there was an alternative?

Thanks!